The element carbon exists in many forms as a bulk material or as deposited films and coatings. Examples include natural diamond and graphite and various forms of diamond and graphite, diamondlike carbon and hard amorphous carbon produced by various vapor deposition processes. A common characteristic of the different forms of elemental carbon is a susceptibility to oxidation at temperatures above approximately 500.degree. C. This susceptibility to oxidation at elevated temperatures is generally a disadvantage which restricts the usefulness of these materials.
Natural diamond and diamond coatings and sheets produced by vapor deposition processes are candidates for applications which can utilize the outstanding combination of mechanical, thermal and optical properties they exhibit. Although diamond is almost chemically inert at low temperatures, it is subject to oxidation at temperatures above approximately 500.degree. C. Oxidation reactions on diamond surfaces generate volatile CO or CO.sub.2 gas and lead to surface erosion and other forms of surface deterioration. The susceptibility to elevated temperature oxidation restricts the use of diamond in some applications where the characteristics of diamond could otherwise be advantageously employed.
As an example, because of its excellent mechanical properties, thermal conductivity, thermal shock resistance and optical characteristics, polycrystalline diamond grown by chemical vapor deposition is considered to be a candidate for use as infrared windows or radomes of high-speed missiles. The diamond might be used to form the window or radome structure or might be employed as a coating over a base of other materials such as germanium, zinc sulfide or zinc selenide. Under some circumstances, the diamond surface of an infrared window or radome of a high-speed missile might reach temperatures in excess of 1000.degree. C. for brief periods during atmospheric flight. The diamond would then be subject to rapid oxidation and associated degradation of its physical and optical properties unless an effective barrier against oxidation were to be provided. Prior attempts have been made to develop suitable oxidation protection by depositing optical coatings of oxidation-resistant materials such as hafnium oxide or yttrium oxide over the diamond. However, due to the inert nature of diamond and to effects such as different rates of thermal expansion of diamond and the coating materials, the deposited coatings have tended to delaminate at elevated temperatures, which is unacceptable. Thin intermediate layers of silicon carbide deposited on the surface of the diamond by sputtering have been used to improve the adhesion of the oxidation protection coatings to the diamond, but the coatings still fail by delamination at unacceptably low temperatures.